1. Technical Field
The invention relates to the fabrication of devices built to small design rules-rules from a few micrometers to fractions of a micrometer. A significant category of contemplated devices is Large Scale Integrated devices; electronic, photonic, as well as hybrid circuits entailing both. Processing of the invention depends upon pattern delineation which expediently accommodates critical contaminant and/or crystallographic perfection demands consistent with further processing requiring reduced pressure or other controlled atmosphere.
2. Description of the Prior Art
The role of Large Scale Integrated circuits in present technology is well understood. Semiconductor LSI is all-pervasive--is at the core of the most significant advances, e.g. computer, communication, etc. Recent advances in photonic elements; for example, in edge-emitting as well as surface-emitting lasers, have brought about intense activity in photonic ("optical") LSI as well as in hybrid circuitry containing both photonic and electronic elements.
Fabrication of devices built to small design rules of a few microns to the most advanced commercial circuits built to rules of one micron or slightly less (LSI chips of a fraction of an inch containing a million or more devices-the "megabit" chip) have depended upon extremely sophisticated advances in many areas.
Fabrication of all such devices is critically dependent on lithographic definition. Reducing design rules have brought about changes in the form of the delineating energy need. Fabrication, once dependent upon radiation in the visible spectrum, has been succeeded by ultraviolet "near" UV (included in the spectral range of from 3000 .ANG. to 4000 .ANG., e.g. using mercury lines at about 3600 .ANG. and 3130 .ANG.). An area of intense worldwide activity directed toward devices built to design rules of 0.5 or 0.6 .mu.m down to perhaps 0.25 .mu.m or smaller, takes the form of prospective substitution of "deep" UV (included in the spectral range of from 1500 .ANG. to 3000 .ANG., e.g. using mercury lines at about 2540 .ANG. and 1810 .ANG.). This technology thought to be suitable for production of a 64 Mbit chip, will in turn yield to still shorter electromagnetic radiation (x-ray, perhaps in the so-called "long" or "soft" wavelength range of from 50 .ANG. to 300 .ANG. or even in the range below 50 .ANG.), or, alternatively, to use of accelerated electrons (either in the form of direct processing by beam-delineation or of masked "flood" illumination). Relevant x-ray and e-flood techniques are described and claimed in co-pending U.S. patent applications Ser. No. 595,341 filed Oct. 10, 1990 and Ser. No. 498,179 filed Mar. 23, 1990.
Pattern delineation, effectively accommodated by energy of reducing wavelength, has resulted, as well, in activity directed toward appropriate actinic (resist) materials for use in otherwise familiar processings as well as toward alternatives. See, Annual Review of Materials Science, vol. 17, pp. 235-271 (1987), "Polymer Materials For Microlithography", E. Reichmanis and L. F. Thompson. Postulated approaches include selectively damaging device-functional material by ion bombardment to result in patterning by selective volatilization or dissolution during "development".
Reducing design rule is accompanied by increasing criticality of a variety of criteria. In particular, demands placed on the various layers of which such devices are constituted become extreme. Extensive activity concerns growth of such layers evidencing necessary perfection--e.g. crystalline perfection as well as prescribed crystalline orientation--and uniformity--compositional as well as dimensional (thickness).
A variety of processes have emerged to meet layer growth requirements. These include Metal Organic Molecular Beam Epitaxy. See "Molecular Beam Epitaxy", M. A. Herman, H. Sitter, Springer-Verlag, 1989 and Metal Organic C hemical Vapor Deposition, and J. P. Hirtz, M. Razeghi, M. Bonnet and J. P. Duchemin in "GaInAsP Alloy Semiconductors", ed. by T. P. Pearsall, J. Wiley & Sons, 1982.
Such vapor deposition processes are necessarily carried out within carefully controlled atmosphere--generally at reduced pressure--and of extreme purity. MOMBE is known to be particularly susceptible to contamination, for example, to adherence-reducing substrate contamination. Chemical residue, for example, residue traceable to resist or resist processing, must be removed to realize the implicit advantages of the deposition process. Requirements become ever-more demanding with still further decrease in delineation wavelength.
Required cleanliness is prospectively thwarted by usual fabrication techniques as applied to state-of-the-art devices. Such fabrication entails deposition of a continuous layer of resist--either positive or negative tone--followed by patterning exposure to selectively decrease or increase rate of removal during development. Development, resulting in evolution of copious quantities of resist, is incompatible with critical deposition steps. Evolution may intefere directly with maintenance of atmosphere, either in terms of composition or of reduced pressure level. Contamination due to resist residue may impact fabrication, e.g. by reducing adherence of subsequently deposited layers, and may otherwise interfere with device functioning.
Such requirements have given rise to fabrication in which pattern delineation is external to controlled atmosphere chambers in which e.g. deposition takes place. While this overcomes one problem, it introduces another. The required breaking of the chamber seal and exposure of pattern-delineated surface to uncontrolled atmosphere gives rise to another form of contamination. Atmospheric contamination may take the form of reaction product, e.g. formation of surface oxide, as well as simple contamination by foreign matter.
Effort to adapt state-of-the-art delineation to such needs are progressing. A particularly useful approach is that of co-pending U.S. patent application, Ser. No. 444,579, filed Nov. 30, 1989, now U.S. Pat. No. 5,106,764. This approach provides for pattern delineation within the controlled atmosphere chamber, thereby permitting vacuum deposition on an uncontaminated surface--on a pattern-delineated surface which has not been exposed to outside atmosphere. Pattern delineation for this earlier "in-situ" procedure depends upon selective removal of extremely thin inorganic resist layers which owe their robustness to chemical bonding to underlying substrate. Promise for this approach is owing to the vastly reduced quantity of released resist material which minimizes interference with processing as well as with ultimate device properties.